205. Isomorphic Strings

Given two strings s and t, determine if they are isomorphic.

Two strings are isomorphic if the characters in s can be replaced to get t.

All occurrences of a character must be replaced with another character while preserving the order of characters. No two characters may map to the same character but a character may map to itself.

For example,
Given "egg", "add", return true.

Given "foo", "bar", return false.

Given "paper", "title", return true.

Note:
You may assume both s and t have the same length.

S: hash map O(n)

注意双向的关系，也可用256长度的数组代替hash map

    bool isIsomorphic(string s, string t) {
        if (s.size() != t.size()) return false;
        unordered_map<char, char> sTot;
        unordered_set<char> tUsed;
        for (int i = 0; i < s.size(); ++i) {
            if (sTot.find(s[i]) == sTot.end()) {
                if (tUsed.find(t[i]) == tUsed.end()) {
                    sTot[s[i]] = t[i];
                    tUsed.insert(t[i]);
                } else {
                    return false;
                }
            } else if (sTot[s[i]] != t[i]) {
                return false;
            }
        }
        return true;
    }